Electron-spin interaction in the spin-Peierls phase of the organic spin chain ( o -DMTTF) 2X ( X =Cl, Br, I)

We investigate the electron spin resonance of the organic spin-Peierls chain ($o$-DMTTF)${}₂X$ with $X$ = Cl, Br, and I. We describe the temperature dependence of the spin gap during the phase transition and quantify the dimerization parameter $\ensuremath{\delta}$. At the lowest temperatures, the susceptibility is governed by defects in the spin dimerized chain. Such strongly correlated defects are the consequence of breaks in the translational symmetry of the chain. In the vicinity of the defects the spins are polarized antiferomagnetically forming a magnetic soliton: a spin $\frac{1}{2}$ quasiparticle of size ruled by $\ensuremath{\delta}$ pinned to the defects. For ($o$-DMTTF)${}₂\mathrm{Br}$ and ($o$-DMTTF)${}₂\mathrm{Cl}$, we show that the one-half of the total number of solitons are in isolation (as singles) whereas the other half form pairs (soliton dimers) with a strong magnetic coupling. The Rabi oscillations of both the single soliton and the soliton dimer are observed, which is a prerequisite in the context of quantum information.